Densification of nitrocellulose



United States Patent DENSIFICATION or NITROCELLULOSE Vernon R. Grassie, Kennett Square, Pa., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Nov. 17, 1958, Ser. No. 774,075

7 Claims. (Cl. 5220) This invention relates to an improved method for producing a nitrocellulose product which is free-flowing and has a high bulk density and which also has improved solubility characteristics. More particularly, the invention relates to the densification of nitrocellulose by using solvent to change the physical form of conventional nitrocellulose from fibrous to granular.

Commercial nitrocellulose is produced and sold today in fibrous form. The bulk density of the product is low, causing relatively high shipping costs. This disadvantage is partially overcome by compressing the nitrocellulose with a ram to increase barrel loadings. As a result of compression in packing, nitrocellulose users find the fibrous material quite diflicult to unload from the barrels. An additional disadvantage of conventional commercial nitrocellulose is the tendency to agglomerate and form large slow-dissolving lumps when added to solvents in certain types of dissolving equipment.

Heretofore attempts to alter the physical form of conventional nitrocellulose with solvents has involved dissolving the nitrocellulose in the solvent and then forming a suspension or emulsion of the resulting nitrocellulose solution in water, after which the solvent is removed by various means such as spray drying, emulsion boiloff, etc. to precipitate the nitrocellulose in fine particle form. However, these prior methods have been costly and tedious, and the product produced is usually of very fine particle size, even dusty. Large amounts of solvent must be employed to prepare the nitrocellulose solutions, on the order of 2.5 to 20 or more parts by weight for each part of nitrocellulose processed, and this necessitates facilities for handling and recovery of large volumes of solvent. Moreover, these prior methods involve several tedious and uneconomical process steps, and while for some purposes a very fine particle size is desirable or necessary, for most applications the very fine, and even dusty, particles produced by these prior methods is neither desirable nor necessary.

-It is an object of this invention, therefore, to produce an improved form of nitrocellulose which can be transported more economically, unloaded from barrels more easily, and which has improved dissolving characteristics.

It is a further object of the invention to provide an improved method for changing the physical form of nitrocellulose from fibrous to granular, which has economic and procedural advantages over prior art methods, and which substantially overcomes the limitations and deficiencies of prior art methods.

These objects and others are accomplished in accordance with the present invention which, generally described, comprises agitating fibrous nitrocellulose in a heated aqueous bath containing organic liquid solvent which has active solvent power for said nitrocellulose and which forms a minimum boiling azeotropic mixture with water, said solvent being present in an amount to soften and destroy the fibrous structure of the nitrocellulose without dissolution thereof, said bath being heated to at least the boiling point of the water-solvent azeotrope,

2,948,601 Patented Aug. 9, 1960 and thereafter removing substantially all of the solvent by distillation while continuing agitation to form smooth, hardened, densified and irregular particles of nitrocellulose.

In a preferred embodiment of the invention, fibrous nitrocellulose is mixed and agitated with water to produce an aqueous slurry of nitrocellulose. Nitrocellulose solvent is then added with agitation to the aqueous nitrocellulose slurry heated to at least the boiling point of the water-solvent azeotrope under reflux conditions in an amount suflicient to destroy the fibrous structure of the nitrocellulose without dissolution thereof, and then removing substantially all of the solvent by distillation while continuing agitation to form smooth, hardened, densified and irregular particles of nitrocellulose. In this process the organic solvent is partitioned to the nitrocellulose whereby it is softened and the fibrous structure thereof is destroyed. However, the nitrocellulose is not dissolved, and the particles thus modified are hardened by boiling off substantially all of the solvent. Agitation is maintained until hardening is complete to prevent agglomeration. The resulting. nitrocellulose product may be recovered in the water-Wet form suitable for shipping, or the water in the end product may be extracted with alcohol to yield an alcohol-wet nitrocellulose suitable for shipping.

The general nature of the invention having been set forth, the following examples are presented as specific illustrations thereof. It will be understood, however, that the invention is not limited to the examples but is susceptible to different modified embodiments which come within the scope of the claims. All parts and percentages given are by weight.

Example 1 One hundred five parts (dry basis) of fibrous waterwet nitrocellulose, 12% nitrogen, second viscosity, were slurried with 945 parts of water. The slurry was heated and agitated in a vapor-tight vessel fitted with a reflux condenser. When the temperature reached 82 C., 111 parts of methyl ethyl ketone were added in two minutes under reflux conditions to serve as organic liquid solvent for the nitrocellulose. A condenser was then added for solvent removal. The agitator peripheral speed was maintained at 1,000 feet per minute as the solvent and water were distilled from the mixture, until the temperature reached C. Thirty minutes were required to complete the distillation. The water-wet product consisted of small smooth, hardened, dense irregular particles. Bulk density was 32.5 pounds of nitrocellulose (dry basis) per cubic foot.

Example 2 One hundred five parts (dry basis) of fibrous waterwet nitrocellulose, 12% nitrogen, /2 second viscosity were slurried with 945 parts of water. The slurry was heated and agitated in a vapor-tight vessel fitted with a reflux condenser. When the temperature reached 82 C., 133 parts of methyl ethyl ketone were added under reflux conditions in three minutes. A condenser was then added for solvent removal. The agitator peripheral speed was maintained at 1,000 feet per minute as the solvent and water were distilled from the mixture until the temperature reached 100 C. Thirty minutes were required to complete the distillation. The water-wet product consisted of smooth, hard, dense irregular particles roughly A3 inch to 4 inch in their greatest dimension. Bulk density was 44.1 pounds of nitrocellulose (dry basis) per cubic foot.

Example 3 One hundred five parts (dry basis) of unjordaned, fibrous water-wet, nitrocellulose, 12.6% nitrogen, 22

seconds viscosity, were slurried with 595 parts of water. The slurry was heated and agitated in a vapor-tight vessel fitted with a reflux condenser. When the temperature reached 100 C., 144 parts of n-butyl acetate were added cooling the mixture to the boiling point of the azeotrope, 90.2 C. A condenser was then added for solvent removal. Agitation was continued as the solvent and water were distilled from the mixture until the temperature reached 100 C. Seventy minutes were required to complete the ditsillation. The water wet product consisted of smooth, hard, dense irregular particles. Bulk density was 42 pounds of nitrocellulose (dry basis) per cubic foot.

Example 4 One hundred five parts (dry basis) of unjordaned, fibrous, water-wet nitrocellulose, 12.6% nitrogen, 22 seconds viscosity, were slurried with 945 parts of Water. The slurry was heated and agitated in a vapor tight vessel fitted with a reflux condenser. When the temperature reached 90 C., 201 parts of methyl isobutyl ketone were added. The solvent which distilled during addition thereof to the heated slurry was returned by reflux to the boiling vessel. The boiling point of the azeotropic mixture was 87.9 C. A condenser was then added for solvent removal. Agitation was continued as the solvent and water were distilled from the mixture until the temperature reached 100 C. Agitator peripheral speed during solvent addition and solvent removal was 628 feet per minute. The water-wet product consisted of small, smooth, dense irregular particles. Bulk density was 40.8 pounds of nitrocellulose per cubic foot.

Example 5 One hundred five parts (dry basis) of jordaned, fibrous, water-wet nitrocellulose, 12.6% nitrogen, 22 seconds viscosity, were slurried with 945 parts of water. The slurry was heated and agitated in a vapor tight vessel equipped with a reflux condenser. When the temperature reached 91 C., 186 parts of methyl isobutyl ketone containing one part of 2-nitrodiphenylamine were added. This addition cooled the mixture to the boiling point of the azeotrope, 87.9 C. A condenser was then added for solvent removal. Agitation was continued as the solvent and water were distilled from the mixture until the temperature reached 100 C. Ninety seven minutes were required to complete the distillation. The waterwet product consisted of smooth, hard, dense irregular particles. Bulk density was 41.7 pounds of nitrocellulose (dry basis) per cubic foot.

Example 6 A 9.5 percnet nitrocellulosezwater slurry containing 1962 lbs. (dry basis) of fibrous RS /2 second nitrocellulose was pumped to a 3900 gallon, vertical, stainless steel jacketed vessel. The vessel was equipped with a water cooled reflux condenser and a six-bladed turbo type agitator driven at a peripheral speed of 1260 feet per minute. The agitated slurry was heated to 80 C. in the vessel with jacket and sparge steam. Two thousand two hundred pounds of methyl ethyl ketone were pumped into the vessel at the rate of 155 pounds per minute. During solvent addition the sparge steam was off, and the jacket steam was used as necessary to maintain the mixture at the boiling point of the azeotrope, 73.5 C. All condensate was returned to the vessel. Solvent was then removed by distillation over a 30 minute period using sparge and jacket steam until the temperature reached 100 C. Agitation was continued throughout solvent addition and solvent removal.

The nitrocellulose thus densified was dropped from the vessel into draining bins, and after the water drained off, denatured ethyl alcohol (2B formula) at 65 C. was passed through the nitrocellulose for 35 minutes with fresh alcohol pumped into the bins at a rate suffici nt to keep the nitrocellulose covered. This was followed by similar treatment with unheated alcohol for 35 minutes. The product was allowed to drain for 15 minutes, and was then packed into barrels. The alcohol-wet product consisted of small, smooth, dense, irregular particles having a bulk density of 31 lbs. nitrocellulose (dry basis) per cubic foot. Alcohol content was 30%.

Example 7 Five thousand grams (dry basis) of water-wet fibrous nitrocellulose of 12.6% nitrogen content, after conventional preliminary stabilization treatment, but before jordaning, were slurried with 45,000 grams of water in a 20-gallon jacketed vessel fitted with a reflux condenser and a 12-inch diameter turbine agitator driven at 200 r.p.m. The slurry was heated to 91 C. by employing steam in the jacket of the vessel. Nine thousand five hundred ten grams of methyl isobutyl ketone were then added to the heated aqueous nitrocellulose slurry under reflux conditions with agitation, requiring about 15 minutes. The solvent was then removed by distillation with continued agitation for minutes, during which time the distillation temperature gradually increased to the boiling point of water. Distillation with agitation was continued for an additional 140 minutes. Substantially all of the solvent was removed by distillation, and the nitrocellulose was transformed, without being dissolved, from its initial fibrous state into smooth, hardened, densified, irregular particles having a bulk density (dry basis) of 41 pounds per cubic foot. Nitrocellulose in its initial fibrous state, derived from either cotton linters or wood pulp, has a bulk density between about 8 and about 15 pounds per cubic foot.

Example 8 One hundred fifty parts (dry basis) of water-wet fibrous nitrocellulose of 12.6% nitrogen content, after conventional preliminary stabilization treatment, but before jordaning, were slurried with 1,350 parts of water in a vessel equipped with an agitator and a reflux condenser. The slurry was heated to C. with agitation, whereupon three hundred eight parts of butyl acetate were added to the heated agitated aqueous slurry of nitrocellulose in about two minutes, thus depressing the temperature of the mixture to about 91 C., the boiling point of the butyl acetate-water azeotrope being 902 C. Substantially all of the solvent was then removed by distillation while continuing heating and agitation until the temperature of the slurry reached 100 C. The nitrocellulose was transformed, without being dissolved, from its initial state into smooth, hardened, densified, irregular particles having a bulk density of 44 pounds per cubic foot.

With reference to all of the above examples, the nitrocellulose products obtained in accordance with this invention consisted of smooth, hardened, dense, irregular particles of nitrocellulose. These particles were from about ,4 inch to about l 4 inch ni their greatest dimension and all commercial types of nitrocellulose can be densified by solvent granulation as set forth herein.

From an economic viewpoint it is desirable to practice the invention with aqueous slurries containing as much fibrous nitrocellulose as practicable, and the upper practical limit of nitrocellulose in the slurry is governed by the ability to agitate the slurry effectively. Generally, nitrocellulose-water slurries containing from about 3% to about 15% by weight of fibrous nitrocellulose have been employed, and preferably from about 7% to about 12% by weight of nitrocellulose. sistencies of less than 3% by weight of nitrocellulose can be employed, it is not presently considered to be economical -to do so. Furthermore, the invention is not limited to a maximum of 15% by Weight of nitroe lulose in the slurry, since the upper practical limit is Although slurry congoverned by the ability to agitate the slurry effectively. The fibrous nitrocellulose to be densified may be jordaned, or otherwise comminuted, if desired, and such comminution generally makes it possible to increase the quantity of nitrocellulose which can be effectively agitated in the slurry. Generally, somewhat smaller densified particles are produced when jordaned nitrocellulose is employed. It will be understood, however, that jordaning, or equivalent comminution is not necessary for the practice of this invention.

Solvent granulation and densification of nitrocellulose in accordance with this invention depends upon the action of an active solvent upon fibrous nitrocellulose suspended in water. The desired action is a softening of the nitrocellulose fibers by the solvent to eliminate the fibrous character thereof, without actually dissolving any measurable amount of the nitrocellulose. Organic liquid solvents suitable for use in practicing this invention are those having an active solvent power for nitrocellulose, and preferably having an appreciable vapor pressure at or below the boiling point of water. The most useful solvents are those which have limited solubility in water and which form a minimum boiling azeotropic mixture with water. Suitable solvents include, by way of example but not in limitation of the invention, various ketones such as methyl ethyl ketone, methyl propyl ketone, methyl isobutyl ketone, and the like, and various esters such as ethyl acetate, propyl acetate, butyl acetate, ethyl butyrate, isopropyl butyrate, ethyl propionate, ,B-ethoxyethyl acetate, and the like. Butyl acetate and methyl isobutyl ketone are presently preferred for densifying nitrocellulose in accordance with this invention. The solvent treatment for densifying nitrocellulose can be accomplished by addition of the solvent to an agitated slurry of nitrocellulose in water, by addition of nitro cellulose to an agitated mixture of water and solvent, or by simultaneous addition of all ingredients, with agitation, as in a continuous process.

In the solvent granulation and densification of nitrocellulose in accordance with this invention, the ratio of active organic liquid nitrocellulose solvent to water is controlled in a range where enough solvent is partitioned to the nitrocellulose to soften and gel the tiny fibrous projections which are characteristic of nitrocellulose fibers without actually dissolving any measurable portion of the nitrocellulose. Too little solvent leaves the physical form of the nitrocellulose unaltered or not sufliciently altered, thus resulting in a product having low bulk density. Too much solvent causes the product to agglomerate into large hard lumps. Still more solvent, of course, causes undesirable dissolving of the nitrocellulose. The desired degree of alteration of the physical structure of the nitrocellulose is to obtain a product which, after solvent removal, consists of smooth, hardened, densified, irregular shaped particles having a bulk density of at least about 25 pounds per cubic foot. It will be apparent that solvent requirement to obtain this objective will vary somewhat with slurry consistency, initial physical form of the nitrocellulose, temperature, degree of agitation, and solvent chosen for densification purposes. In general, however, it has been found that suitable products for the purposes of this invention are obtained by using between about 15% and about 22% of either butyl acetate or methyl isobutyl-ketone, based on combined weight of solvent and water in the aqueous slurry, and preferably between about 16% and 19% by weight. The minimum suitable quantity of methyl ethyl ketone is on the order of approximately one part by weight of methyl ethyl ketone for each 2.7 parts of water in the aqueous nitrocellulose slurry. Similarly, the minimum suitable quantity of ethyl acetate is on the order of approximately one part by weight of ethyl acetate for each 9.5 parts of water in the aqueous nitrocellulose slurry. However, regardless of the solvent employed for densification purposes, it is a simple expedient to carry out a preliminary densification trial, using the hereinabove indicated quantities as a guide, and determine the degree of densification obtained, based on bulk density requirements. It will be apparent, of course, that densification as measured by increased bulk density of the densified product, will improve with increasing quantity of active nitrocellulose solvent employed, until that point is reached where the softened particles beging to agglomerate into large lumps. Thus, this invention provides a basis for density control in the products produced. The phenomenon of agglomeration of the softened particles into large lumps governs the upper useful limit of active solvent which can be employed to alter the physical structure of the nitrocellulose in accordance with this invention.

In practicing this invention, it has been found that softening of the nitrocellulose fibers by the active nitrocellulose solvent occurs very rapidly upon bringing the nitrocellulose and solvent into contact With each other in the aqueous slurry. Accordingly, it is important to maintain vigorous agitation in the aqueous slurry from the moment that the nitrocellulose is contacted by the active solvent in the aqueous slurry until the softened nitrocellulose particles have been hardened and densified by removal of substantially all of the solvent. Agitation prevents substantial agglomeration of the softened nitrocellulose particles into lumps.

Any desired additives, such as nitrocellulose stabilizers, plasticizers, or other desirable additives, which are soluble in the solvent used for densification and which are insoluble in water can be introduced with the solvent and become very uniformly distributed into the nitrocellulose product during the densification process of this invention. Example 5 exemplifies this practice.

In the preferred practice of this invention, the solvent is added to the aqueous nitrocellulose slurry which is heated at least to the boiling point of the water-solvent azeotrope, preferably under reflux conditions, since the partition of solvent to the nitrocellulose is aided by reduced solubility of the solvent in water. Moreover, there is no tendency for the softened particles of nitrocellulose to pulf up or popcorn during subsequent solvent removal, thus aiding materially in attaining a high bulk density in the densified nitrocellulose product.

Hardening of the softened nitrocellulose is carried out by boiling off the solvent until substantially all solvent 1s removed from the nitrocellulose. This may require from about 20 minutes to about 4 hours, depending upon equipment used and the solvent which has been employed. Efficient recovery of solvent for re-use in the process is necessary for economical operation, and is readily achieved by distillation.

Following solvent removal, the water associated with the product normally must be replaced by an alcohol such as ethyl-, isopropyl-, or butyl alcohol. This may be accomplished by an extraction process in a column or bin. Advantage is taken of the high density and free-draining characteristics of the solvent granulated nitrocellulose to eliminate the hydraulic presses or contrifuges normally needed to reduce the volatile content after dehydration with alcohol. Satisfactory volatile contents are obtained after a 5 to 15 minute draining period when granular nitrocellulose is dehydrated.

The granular nitrocellulose product of this invention is composed of small, dense, irregular particles which when magnified appear to have smooth, glazed surfaces lacking the fibrous projections which give conventional fibrous nitrocellulose low bulk density and high compressibility. Generally, the densified granular nitrocellulose product will consist of particles having a relatively wide diversity of sizes, as determined by screen analysis. It is presently believed that this is a desirable feature of the densified nitrocellulose since the smaller particles help to fill in the spaces between larger particles, thus leading to a product of higher bulk density.

It has been found, however, that the densified nitrocellulose product of this invention can be comminuted to any desired particle size by wet milling, as for example, in a jordan mill, without losing the desired properties of high bulk density, free flowing character and improved solubility. Such wet milled comminuted material has particular utility in end use applications which involve slurry or suspension formulation techniques, as for example, in hydrosol, organosol, and plastisol formulation, in multicolor lacquer suspension formulation and other techniques including application for explosives where the product in slurry or suspension formulations has been found advantageous.

The densified material, in either water-wet or alcoholwet form, flows freely, is relatively incompressible, and has a bulk density of about 25 to about 45 pounds of nitrocellulose (dry basis) per cubic foot. The chemical characteristics of the granular nitrocellulose product are apparently the same as conventional fibrous nitrocellulose.

The higher bulk density of the granular nitrocellulose products of this invention results in transportation savings. Commercial RS /2 second nitrocellulose, derived from wood pulp, for example, has a loose bulk density of about 12 pounds (dry basis) per cubic foot and is rammed into barrels to a density of 23 pounds per cubic foot. Granular nitrocellulose with a bulk density on the order of 30 to 40 pounds per cubic foot, for example, permits substantially heavier barrel loadings than previously possible with conventional fibrous nitrocellulose. Furthermore, granular nitrocellulose can be poured from the barrels for very easy unloading. Present fibrous nitrocellulose must be dug out of the barrel. Data show that the granular nitrocellulose of this invention dissolves to form lacquer 1.5 to 12 times as rapidly as conventional fibrous nitrocellulose, depending upon the type of agitation used. The poorer the agitation, the greater is the advantage of granular nitrocellulose. The elimination of the costly and labor-consuming pressing or centrifuging and block breaking operations usually following nitrocellulose dehydration is an important advantage of granular nitrocellulose. Moreover, since the process of this invention merely softens the nitrocellulose fibers without dissolving any measurable portion of the nitrocellulose, it is apparent that much less solvent is employed than in processes which involve dissolving the nitrocellulose. Furthermore, it is apparent that the process of this invention is simpler and less tedious than the processes which involve dissolving the nitrocellulose, making an emulsion or suspension of the resulting nitrocellulose solution in Water, and then precipitating the nitrocellulose from the emulsified or suspended solution droplets.

The free-flowing, fast-dissolving and high bulk density nitrocellulose products of this invention can be used in any application where commercial nitrocellulose is now 8 used, such as lacquers, plastics, paints, adhesives, coatings, impregnations, propellants, and the like.

This application is a continuation-in-part of my application, Serial No. 556,058, filed December 29, 1955, which in turn is a continuation-in-part of my application Serial No. 448,156, filed August 5, 1954, and which is now abandoned.

What I claim and desire to protect by Letters Patent is:

1. A process for densifying nitrocellulose which comprises contacting fibrous nitrocellulose with agitation in a heated aqueous bath with an organic liquid solvent Which has active solvent power for said nitrocellulose and which forms a minimum boiling azeotropic mixture with water, said solvent being present in an amount to soften and destroy the fibrous structure of said nitrocellulose without dissolution thereof, said bath being heated to at least the boiling point of the water-solvent azeotrope during the entire period of contact between said nitrocellulose and said solvent, and thereafter removing substantially all of the solvent by distillation while continuing agitation to form smooth, hardened, densified and irregular particles of partially colloided nitrocellulose.

2. The process in accordance with claim 1 in which the organic liquid solvent is ethyl acetate.

3. The process in accordance with claim 1 in which the organic liquid solvent is butyl acetate.

4. The process in accordance with claim 1 in which the organic liquid solvent is methyl ethyl ketone.

5. The process in accordance with claim 1 in which the organic liquid solvent is methyl isobutyl ketone.

6. A process for densifying nitrocellulose which comprises forming a slurry of fibrous nitrocellulose in water, adding nitrocellulose solvent to the aqueous nitrocellulose slurry in an amount sufficient to destroy the fibrous structure of the nitrocellulose without dissolution thereof, said solvent forming a minimum boiling azeotropic mixture with water and said slurry being heated to at least the boiling point of the water-solvent azeotrope during addition of said solvent, and thereafter removing substantially all of the solvent by distillation to form smooth, hardened, densified and irregular particles of nitrocellulose, agitation being maintained throughout the process.

7. The process in accordance with claim 6 in which the nitrocellulose solvent is added to the heated aqueous nitrocellulose slurry under reflux conditions.

References Cited in the file of this patent UNITED STATES PATENTS 2,027,114 Olsen et a1. Jan. 7, 1936 2,160,626 Schaefer May 30, 1939 2,235,298 Olsen Mar. 18, 1941 2,292,469 Olsen Aug. 11, 1942 

1. A PROCESS FOR DENSIFYING NITROCELLULOSE WHICH COMPRISES CONTACTING FIBROUS NITROCELLULOSE WITH AGITATION IN A HEATED AQUEOUS BATH WITH AN ORGANIC LIQUID SOLVENT WHICH HAS ACTIVE SOLVENT POWER FOR SAID NITROCELLULOSE AND WHICH FORMS A MINIMUM BOILING AZEOTROPIC MIXTURE WITH WATER, SAID SOLVEDNT BEING PRESENT IN AN AMOUNT TO SOFTEN AND DESTROY THE FIBROUS STRUCTURE OF SAID NITROCELLULOSE WITHOUT DISSOLUTION THEREOF, SAID BATH BEING HEATED TO AT LEAST THE BOILING POINT OF THE WATER-SOLVENT AZEOTROPE DURING THE ENTIRE PERIOD OF CONTACT BETWEEN SAID NITROCELLULOSE AND SAID SOLVENT, AND THEREAFTER REMOVING SUBSTANTIALLY ALL OF THE SOLVENT BY DISTILLATION WHILE CONTINUING AGITATION TO FORM SMOOTH, HARDENED, DENSIFIED AND IRREGULAR PARTICLES OF PARTIALLY COLLOIDED NITROCELLULOSE. 